1. Field of the Invention
This invention relates to a semiconductor device using nitride compound semiconductors and, in particular, to a gallium nitride blue-violet semiconductor laser and its manufacturing method.
2. Description of the Related Art
Galium nitride compound semiconductors, such as GaN, InGaN, GaAlN and InGaAlN, are recently remarked as materials of short-wavelength semiconductor lasers for use in high-density optical disc systems because the laser light with a short wavelength can be tightly focused and is therefore effective for increasing the recording density.
In general, in order to ensure the emission of semiconductor lasers with a low threshold current, carrier injection to the active layer must be performed effectively. For this purpose, it is important to form a hetero-structured p-n junction and a current confining structure. Additionally, semiconductor lasers for use as a light source of a pickup-head in an optical disc system must be regulated in oscilation mode. Therefore, a structure is required to obtain a stable emission in the fundamental oscilation mode by increasing the differences in refractive index between the active layer and the neighboring layers, and by appropriately absorbing laser light swerving from the active layer to decrease gains of higher-order modes.
However, in the case of nitride compound semiconductor lasers, there has been no report except for so-called gain-guiding lasers in which a ridge structure is employed only for a stripe-shaped electrode or a p-type semiconductor layer. On the other hand, it is known from semiconductor lasers using InGaAlP or other conventional materials that an active layer of a multi-quantum well structure is better than a bulk active layer to decrease the threshold current density. As for nitride compound semiconductor lasers, however, although multi-quantum well structures have been originally used as the active layer, power consumption is still large because they need a high threshold current and a high operation voltage. It results in too short laser lives and too large noise of oscillated laser light to use the lasers in optical disc systems. Moreover, since the maximum optical output of conventional gain-guided semiconductor lasers is on the order of mW, they are insufficient for use as a high-power light source available for 30 mW required for deletion and recording in optical disc systems.
This is caused by the difficulty in carrier injection into the quantum well active layer with a high density and a high efficiency, the difficulty in making a structure controllable in the fundamental transverse mode for low noise characteristics, and the difficulty in integrating a plurality of semiconductor lasers. That is, it has been difficult to inject carriers into the active layer with a high current density necessary for the fundamental transverse mode by confining and guiding the current from the electrode into the active layer without deviation from the active layer while making an appropriate absorption region for obtaining only the fundamental transverse mode. Moreover, integration of a plurality of nitride compound semiconductor lasers has been difficult due to the difficulty in alternately forming current confining regions and absorption regions.
As discussed above, for actual application of lasers to optical disc systems, or the like, it is necessary to realize a highly reliable gallium nitride blue-violet semiconductor laser promising continuous emission in the fundamental transverse mode with a low threshold current and low voltage.
Conventional semiconductor lasers using nitride compound semiconductors are not good in noise characteristics because of the difficulty in making a current confining structure controllable in the fundamental transverse mode. Additionally, because of the difficulty of high-density carrier injection into the active layer, it has been difficult to realize a device for a high optical output with a low threshold current and a low operation voltage.